The field of the invention relates generally to apparatus, systems, and methods for trace detection, and more particularly, to thermal desorption of trace particles to facilitate detection.
Some known detection systems detect trace materials from a swab that is used to sample trace particles from luggage, clothing, or other sources of trace particles. The swab is inserted into a small heated desorber, and the heat from the desorber changes the phase of the sampled trace particles from solid to vapor. The vapor is then channeled into a detector, which analyzes the chemical and/or biological composition of the vapor.
At least some known detection systems incorporate flash heating to vaporize trace particles. Some known flash heaters include one or more planar foil heating elements, each of which is etched to form a serpentine conducting path of essentially constant width. The conductive path is generally designed to have low resistance and a large width relative to its thickness to facilitate an increased heat output. The temperature of such etched-foil flash heater elements can increase by, for example, approximately 100 degrees Celsius or more in a few seconds.
At least some known detection systems use such a flash heater to quickly step a desorber from a first temperature, corresponding to a vaporization temperature of a first trace material of interest, to a second temperature, corresponding to a vaporization temperature of a second trace material of interest. However, such rapid flash heating typically causes some locations of the heating element surface to heat more quickly than others, causing localized hot spots and large temperature gradients across the heating element surface. Collected trace particles on portions of the swab adjacent to relatively hot or cool spots on the heating element may not vaporize within an expected time period or may degrade too quickly, resulting in decreased quality and consistency of detection.
At least some known heating elements suitable for flash heaters have attempted to limit non-uniformity in the temperature of the heating element by limiting an effect termed “current crowding.” Current crowding refers to the tendency for electrical currents, which follow the path of minimum resistance, to crowd around the inside of each bend in a serpentine foil heating element, similar to runners taking the shortest path around a curve on a track. Thus, current crowding creates relatively hot spots around the inside of the bends and relatively cool spots around the outside of the bends. Some known heating elements have attempted to limit the temperature gradient across the bend by etching a single, relatively wide serpentine foil element into multiple parallel narrower elements, distributing the current crowding effect among each parallel current path. However, this attempt to limit current crowding does not mitigate other factors that lead to a large temperature gradient across known serpentine etched-foil heating elements. One such factor is that the peripheral edges of the foil heating element are free to transfer heat to the environment in directions both normal to the foil surface and tangential to the foil surface, while the inner portions of the heating element can only transfer heat to the environment in a direction normal to the foil surface. As a result, a large temperature gradient may develop between the relatively hotter center of the element and the relatively cooler periphery of the element when the flash heater is activated.